Allow to overwrite the auth `secret_key = "MaxVerstappenWC2021"` with env var

[josecelano]

In the auth config section you need to define a secret:

[auth]
email_on_signup = "Optional"
min_password_length = 6
max_password_length = 64
secret_key = "MaxVerstappenWC2021"

Allow to overwrite it with an env var like the TORRUST_INDEX_TRACKER_API_TOKEN

# Please override the tracker token setting the
#    `TORRUST_INDEX_TRACKER_API_TOKEN`
#                     environmental variable!

The name could be TORRUST_INDEX_AUTH_SECRET_KEY.

In general env vars could be like TORRUST_CONFIG_SECTION_VAR_NAME.

• Prefix: TORRUST_INDEX_
• Configuration section: AUTH, DATABASE, WEBSITE, TRACKER, NET, ...
• Config option in uppercase: SECRET_KEY